[1] https://en.wikipedia.org/wiki/List_of_killings_by_law_enforc...
RCMP members are normally trained for situations that local police (that specialize in de-escalation) can't handle - but when it comes to reservations they're the authority in pretty much all cases. Being in the RCMP isn't an easy job and it'll leave you scarred and hardened in a way that can seriously impede your ability to properly deescalate non-violent situations.
First Nations deal with the (federal) Crown, and so the RCMP is the federal law enforcement agency. Unless (a/each) First Nation wants to run their own police force internally?
But there are entire regions that don't do that because of the costs involved, and so they contract out to the RCMP to be the provincial (and even municipal) police.
The larger provinces (ON, QC) have their own services, and they are also contracted out to the smaller municipalities. Not sure how provincial police would work (or not) on the reserves.